Traditionally, solder bump was used on the lead pad of IC chip on wafers for flip chip assembly. However, copper (Cu) pillar bump gradually takes the place of the solder bump as a next generation flip chip interconnect which offers advantages in many designs while meeting current and future ROHS requirements. During electroplating of Cu pillar, a voltage drop variation typically exists along an irregular surface of a substrate which can result in an uneven metal deposit on the substrate. Some parts of the substrate would have been overplated while other parts would have been underplated.
In order to solve the problems, plating levelers could be added into the electroplating bath in order to achieve a uniform metal deposit on a substrate surface.
CA1108087 discloses a method and bath for the electrodeposition of bright to semi-bright zinc plate, wherein there is incorporated into the bath a water soluble additive which is a polymer derived from polyepichlorohydrin or polyepibromohydrin and a tertiary amine and wherein a quaternary group +NR3—replaces at least 25 percent of the halide groups of the polyhalohydrin. In the quaternary ammonium group, X is a chloride or bromide group and R is an alkyl, alkenyl, alkynyl or alkanol radical or mixtures thereof, each radical containing from 1 to 4 carbon atoms.
U.S. Pat. No. 4,555,315 discloses an improved electrolyte composition and process for electrodepositing bright, level and ductile copper deposits on a substrate. A constituent of the additive system comprises a bath soluble adduct of a tertiary alkyl amine with polyepichlorohydrin bath soluble adduct of a tertiary alkyl amine with polyepichlorohydrin.
U.S. Pat. No. 6,610,192 discloses a method of electroplating copper on an integrated circuit substrate having ≤2 μm apertures comprising the steps of contacting the substrate to be plated with a copper electroplating bath comprising one or more leveling agents. The leveling agent is a reaction product of a heterocyclic amine with an epihalohydrin.
U.S. Pat. No. 7,662,981 discloses a leveler compound for depositing metal layers using plating baths. The leveler is a reaction product of an amine with a polyepoxide. U.S. Pat. No. 8,114,263 discloses a polyvinylammonium compound for electrolytically depositing a copper deposit. US2010/0126872 discloses a leveler compound being a reaction product of a dipyridyl compound and an alkylating agent. US2013/0068626 discloses a leveling agent comprising a linear or branched, polymeric imidazolium compound.
Although the conventional levelers may improve the quality of metal deposit done by electroplating on the substrate surface, they are still not able to meet the tight requirements of height uniformity for electroplating of copper pillars with different diameters in the recent IC technology. Poor height uniformity of copper pillar can significantly affect the electric conductivity and stability of the wafer assembly since connection bonding between the two copper bumps of two wafers may not be fully connected and copper pillar pairs with lower height may not be well contacted.
There is a need in the art to have a plating additive for electroplating of copper pillars with different diameters, which provides good surface flatness and height uniformity.